iio: ssp_sensors: Convert to use GPIO descriptors

These three GPIO lines used by the Samsung sensor hub is pretty
straight-forward to convert to use GPIO descriptors.

Cc: Karol Wrona <k.wrona@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>